If you are open to using boost, then I would suggest you use the file_lock class. This means you want to keep the filename of the files you open/close because fstream does not do so for you.
They have two modes lock() that you can use for writing (i.e. only one such lock at a time, the sharable lock prevents this lock too) and lock_sharable() that you can use for reading (i.e. any number of threads can obtain such a lock).
Note that you will find it eventually complicated to manage both, read and write, in this way. That is, if there is always someone to read, the sharable lock may never get released. In that case, the exclusive lock will never be given a chance to take....
// add the lock in your class
#include <boost/interprocess/sync/file_lock.hpp>
class my_files
{
...
private:
...
boost::file_lock m_lock;
};
Now when you want to access a file, you can lock it one way or the other. If the thread is in charge of when they do that, you could add functions for the user to have access to the lock. If your implementation of the read and write functions in my_files are in charge, you want to get a stack based object that locks and unlocks for you (RAII):
class safe_exclusive_lock
{
public:
safe_exclusive_lock(file_lock & lock)
: m_lock_ref(lock)
{
m_lock_ref.lock();
}
~safe_exclusive_lock()
{
m_lock_ref.unlock();
}
private:
file_lock & m_lock_ref;
};
Now you can safely lock the file (i.e. you lock, do things that may throw, you always unlock before exiting your current {}-block):
ssize_t my_files::read(char *buf, size_t len)
{
safe_exclusive_lock guard(m_lock);
...your read code here...
return len;
} // <- here we get the unlock()
ssize_t my_files::write(char const *buf, size_t len)
{
safe_exclusive_lock guard(m_lock);
...your write code here...
return len;
} // <- here we get the unlock()
The file_lock uses a file, so you will want to have the fstream file already created whenever the file_lock is created. If the fstream file may not be created in your constructor, you probably will want to transform the m_lock variable in a unique pointer:
private:
std::unique_ptr<file_lock> m_lock;
And when you reference it, you now need an asterisk:
safe_exclusive_lock guard(*m_lock);
Note that for safety, you should check whether the pointer is indeed allocated, if not defined, it means the file is not open yet so I would suggest you throw:
if(m_lock)
{
safe_exclusive_lock guard(*m_lock);
...do work here...
}
else
{
throw file_not_open();
}
// here the lock was released so you cannot touch the file anymore
In the open, you create the lock:
bool open(std::string const & filename)
{
m_stream.open(...);
...make sure it worked...
m_lock.reset(new file_lock(filename));
// TODO: you may want a try/catch around the m_lock and
// close the m_stream if it fails or use a local
// variable and swap() on success...
return true;
}
And do not forget to release the lock object in your close:
void close()
{
m_lock.reset();
}
